Summary
Concentrations of ground‐level ozone (O3) over much of the Earth's land surface have more than doubled since pre‐industrial times. The air pollutant is highly variable over time and space, ...which makes it difficult to assess the average agronomic and economic impacts of the pollutant as well as to breed crops for O3 tolerance. Recent modeling efforts have improved quantitative understanding of the effects of current and future O3 on global crop productivity, and experimental advances have improved understanding of the cellular O3 sensing, signaling and response mechanisms. This work provides the fundamental background and justification for breeding and biotechnological approaches for improving O3 tolerance in crops. There is considerable within‐species variation in O3 tolerance in crops, which has been used to create mapping populations for screening. Quantitative trait loci (QTL) for O3 tolerance have been identified in model and crop species, and although none has been cloned to date, transcript profiling experiments have identified candidate genes associated with QTL. Biotechnological strategies for improving O3 tolerance are also being tested, although there is considerable research to be done before O3‐tolerant germplasm is available to growers for most crops. Strategies to improve O3 tolerance in crops have been hampered by the lack of translation of laboratory experiments to the field, and the lack of correlation between visual leaf‐level O3 damage and yield loss to O3 stress. Future efforts to screen mapping populations in the field and to identify more promising phenotypes for O3 tolerance are needed.
Significance Statement
Concentrations of ozone, the most damaging air pollutant to crops, have doubled since pre‐industrial times. Here I outline the challenges that ozone pollution imposes on agriculture, describe recent advances in understanding its global impact on agriculture, discuss how it signals and damages plants, and describe efforts to improve ozone tolerance in crops.
Free‐air CO2 enrichment (FACE) allows open‐air elevation of CO2 without altering the microclimate. Its scale uniquely supports simultaneous study from physiology and yield to soil processes and ...disease. In 2005 we summarized results of then 28 published observations by meta‐analysis. Subsequent studies have combined FACE with temperature, drought, ozone, and nitrogen treatments. Here, we summarize the results of now almost 250 observations, spanning 14 sites and five continents. Across 186 independent studies of 18 C3 crops, elevation of CO2 by ca. 200 ppm caused a ca. 18% increase in yield under non‐stress conditions. Legumes and root crops showed a greater increase and cereals less. Nitrogen deficiency reduced the average increase to 10%, as did warming by ca. 2°C. Two conclusions of the 2005 analysis were that C4 crops would not be more productive in elevated CO2, except under drought, and that yield responses of C3 crops were diminished by nitrogen deficiency and wet conditions. Both stand the test of time. Further studies of maize and sorghum showed no yield increase, except in drought, while soybean productivity was negatively affected by early growing season wet conditions. Subsequent study showed reduced levels of nutrients, notably Zn and Fe in most crops, and lower nitrogen and protein in the seeds of non‐leguminous crops. Testing across crop germplasm revealed sufficient variation to maintain nutrient content under rising CO2. A strong correlation of yield response under elevated CO2 to genetic yield potential in both rice and soybean was observed. Rice cultivars with the highest yield potential showed a 35% yield increase in elevated CO2 compared to an average of 14%. Future FACE experiments have the potential to develop cultivars and management strategies for co‐promoting sustainability and productivity under future elevated CO2.
Lessons from more than 250 experiments, with 15 field crops on five continents using free‐air CO2 enrichment (FACE). Plus, its combination with drought and nitrogen, and with rising ozone and temperature.
Rice is arguably the most important food source on the planet and is consumed by over half of the world's population. Considerable increases in yield are required over this century to continue ...feeding the world's growing population. This meta-analysis synthesizes the research to date on rice responses to two elements of global change, rising atmospheric carbon dioxide concentration (CO₂) and rising tropospheric ozone concentration (O₃). On an average, elevated CO₂ (627 ppm) increased rice yields by 23%. Modest increases in grain mass and larger increases in panicle and grain number contributed to this response. The response of rice to elevated CO₂ varied with fumigation technique. The more closely the fumigation conditions mimicked field conditions, the smaller was the stimulation of yield by elevated CO₂. Free air concentration enrichment (FACE) experiments showed only a 12% increase in rice yield. The rise in atmospheric CO₂ will be accompanied by increases in tropospheric O₃ and temperature. When compared with rice grown in charcoal-filtered air, rice exposed to 62 ppb O₃ showed a 14% decrease in yield. Many determinants of yield, including photosynthesis, biomass, leaf area index, grain number and grain mass, were reduced by elevated O₃. While there have been too few studies of the interaction of CO₂ and O₃ for meta-analysis, the interaction of temperature and CO₂ has been studied more widely. Elevated temperature treatments negated any enhancement in rice yield at elevated CO₂, which suggests that identifying high temperature tolerant germplasm will be key to realizing yield benefits in the future.
This review summarizes current understanding of the mechanisms that underlie the response of photosynthesis and stomatal conductance to elevated carbon dioxide concentration (CO₂), and examines how ...downstream processes and environmental constraints modulate these two fundamental responses. The results from free-air CO₂ enrichment (FACE) experiments were summarized via meta-analysis to quantify the mean responses of stomatal and photosynthetic parameters to elevated CO₂. Elevation of CO₂ in FACE experiments reduced stomatal conductance by 22%, yet, this reduction was not associated with a similar change in stomatal density. Elevated CO₂ stimulated light-saturated photosynthesis (Asat) in C₃ plants grown in FACE by an average of 31%. However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis at elevated CO₂ had greater potential for increases in Asat than those where photosynthesis became ribulose-1,5-bisphosphate (RubP)-limited at elevated CO₂. Both nitrogen supply and sink capacity modulated the response of photosynthesis to elevated CO₂ through their impact on the acclimation of carboxylation capacity. Increased understanding of the molecular and biochemical mechanisms by which plants respond to elevated CO₂, and the feedback of environmental factors upon them, will improve our ability to predict ecosystem responses to rising CO₂ and increase our potential to adapt crops and managed ecosystems to future atmospheric CO₂.
Tropospheric ozone (O(3)) is a global air pollutant that causes billions of dollars in lost plant productivity annually. It is an important anthropogenic greenhouse gas, and as a secondary air ...pollutant, it is present at high concentrations in rural areas far from industrial sources. It also reduces plant productivity by entering leaves through the stomata, generating other reactive oxygen species and causing oxidative stress, which in turn decreases photosynthesis, plant growth, and biomass accumulation. The deposition of O(3) into vegetation through stomata is an important sink for tropospheric O(3), but this sink is modified by other aspects of environmental change, including rising atmospheric carbon dioxide concentrations, rising temperature, altered precipitation, and nitrogen availability. We review the atmospheric chemistry governing tropospheric O(3) mass balance, the effects of O(3) on stomatal conductance and net primary productivity, and implications for agriculture, carbon sequestration, and climate change.
Non-structural phenolic compounds perform a variety of functions in plants, including acting as antioxidants. We describe a microplate-adapted colorimetric total phenolics assay that utilizes ...Folin-Ciocalteu (F-C) reagent. The F-C assay relies on the transfer of electrons in alkaline medium from phenolic compounds to phosphomolybdic/phosphotungstic acid complexes, which are determined spectroscopically at 765 nm. Although the electron transfer reaction is not specific for phenolic compounds, the extraction procedure eliminates approximately 85% of ascorbic acid and other potentially interfering compounds. This assay is performed in microcentrifuge tubes and assessed in a 96-well plate reader. At least 64 samples can be processed in 1 d.
The current trajectory for crop yields is insufficient to nourish the world's population by 2050
. Greater and more consistent crop production must be achieved against a backdrop of climatic stress ...that limits yields, owing to shifts in pests and pathogens, precipitation, heat-waves and other weather extremes. Here we consider the potential of plant sciences to address post-Green Revolution challenges in agriculture and explore emerging strategies for enhancing sustainable crop production and resilience in a changing climate. Accelerated crop improvement must leverage naturally evolved traits and transformative engineering driven by mechanistic understanding, to yield the resilient production systems that are needed to ensure future harvests.
We quantitatively evaluated the effects of elevated concentration of ozone (O₃) on growth, leaf chemistry, gas exchange, grain yield, and grain quality relative to carbon-filtered air (CF) by means ...of meta-analysis of published data. Our database consisted of 53 peer-reviewed studies published between 1980 and 2007, taking into account wheat type, O₃ fumigation method, rooting environment, O₃ concentration (O₃), developmental stage, and additional treatments such as drought and elevated carbon dioxide concentration (CO₂). The results suggested that elevated O₃ decreased wheat grain yield by 29% (CI: 24-34%) and aboveground biomass by 18% (CI: 13-24%), where CI is the 95% confidence interval. Even in studies where the O₃ range was between 31 and 59 ppb (average 43 ppb), there was a significant decrease in the grain yield (18%) and biomass (16%) relative to CF. Despite the increase in the grain protein content (6.8%), elevated O₃ significantly decreased the grain protein yield (-18%). Relative to CF, elevated O₃ significantly decreased photosynthetic rates (-20%), Rubisco activity (-19%), stomatal conductance (-22%), and chlorophyll content (-40%). For the whole plant, rising O₃ induced a larger decrease in belowground (-27%) biomass than in aboveground (-18%) biomass. There was no significant response difference between spring wheat and winter wheat. Wheat grown in the field showed larger decreases in leaf photosynthesis parameters than wheat grown in < 5 L pots. Open-top chamber fumigation induced a larger reduction than indoor growth chambers, when plants were exposed to elevated O₃. The detrimental effect was progressively greater as the average daily O₃ increased, with very few exceptions. The impact of O₃ increased with developmental stages, with the largest detrimental impact during grain filling. Both drought and elevated CO₂ significantly ameliorated the detrimental effects of elevated O₃, which could be explained by a significant decrease in O₃ uptake resulting from decreased stomatal conductance.
Free-air CO2 enrichment (FACE) experiments allow study of the effects of elevated CO2 on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer-reviewed ...articles describing physiology and production in the 12 large-scale FACE experiments (475-600 ppm) were collected and summarized using meta-analytic techniques. The results confirm some results from previous chamber experiments: light-saturated carbon uptake, diurnal C assimilation, growth and above-ground production increased, while specific leaf area and stomatal conductance decreased in elevated CO2. There were differences in FACE. Trees were more responsive than herbaceous species to elevated CO2. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated CO2 may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric CO2; but even with FACE there are limitations, which are also discussed.
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